Page:The Journal of geology (IA journalofgeology21894univers).pdf/405

and unimportant oscillations, from that time until now. The beginning or earliest known stage of the progressive elevatory wave probably thus raised the northern half of the Mississippi basin to a variable amount ranging from 100 feet or less to 500 feet or more. It was practically completed, for this area, previous to the accumulation of the outer and earlier moraines in the series of many which mark pauses in the further recession of the ice-sheet. Thenceforward the glacial melting appears to have been more rapid than before, giving to the ice steeper frontal gradients whereby its drift was amassed more commonly in hills, ridges, and lake-enclosing hollows, and especially in the very irregularly knolly and hilly moraine belts.

The rapidity of the glacial recession and of this ensuing epeirogenic uplift in its wave-like advance upon the area of the glacial Lake Agassiz, extending nearly 700 miles from south to north in the basin of the Red river and of Lake Winnipeg, surpasses all previous knowledge in what it reveals concerning the mobility of the earth's crust. The postglacial duration of Lake Michigan and its companion great lakes of the St. Lawrence has been shown, by numerous independent but well agreeing observations and estimates, to be no longer than 6,000 to 10,000 years. Now the amount of wave erosion on the shores of Lake Michigan and the resulting accumulation of beach sand, heaped into dunes upon large areas about the south end of the lake, must exceed, by a ratio of 10:1 or 20:1, the corresponding wave action in its total amount at all the successive levels held by Lake Agassiz during its history, which accordingly must be comprised within some such time as 1,000 years or perhaps less.^[1] During this geologically very short timee, the ice was melted away upon the distance of 700 to 1,000 miles from the middle of the west side of Minnesota to James and Hudson bays, and the Lake Agassiz basin was differentially uplifted mostly 300 to 500 feet, to the height which it has ever since retained without appreciable later change. To understand the wave-like devel-